In what we know now as Nubia, Kush used to be located
At first the relation between this two civilizations was only of trade, however as time progressed and changes came, it became more complicated due to several social, economic, political and even cultural reasons. Through history, they had several confrontations and their relation became really conflictive
As an example, for a long time Kush outpowered Egypt and ruled its land. But on the other hand, in 1500 BC Kush was conquered by Egypt.